The present invention generally relates to synchronizing devices for laser printers, and more particularly to a synchronizing device for controlling exposure positions in a laser printer.
In a laser printer which prints image information by use of a scanning laser beam and especially in a laser printer which uses a plurality of scanning laser beams for the printing, it is important from the point of view of maintaining a high picture quality that print starting positions where each of the laser beams start to print the image information are accurately controlled to coincide with each other. Conventionally, a photodetector is provided at such a position outside an image forming region in which the scanning laser beam prints the image information, so that with each scan the scanning laser beam scans the position of the photodetector before scanning a photosensitive drum. The photodetector outputs a signal when scanned by the laser beam, and a counter starts to count pulses of a master clock signal responsive to this signal from the photodetector. When a counted value of the counter reaches a preset value, the laser beam is controlled to start printing the image information from the print starting position. Hence, the laser beam is always controlled to start the printing from the same print starting position for each scan.
However, when a plurality of laser beams which scan respective photosensitive drums are used for the printing, it is necessary to provide a photodetector with respect to each of the laser beams. In this case, a slight error may occur in the mounting positions of the photodetectors and optical systems. When the mounting error corresponds a time interval which is an integral multiple of the period of the master clock signal, it is possible to control the print starting positions of each of the laser beams to coincide with each other by adjusting the preset value of the counter. But it is impossible to match the print starting positions of the laser beams by simply adjusting the preset value of the counter when the mounting error corresponds to a time interval which is within one period of the master clock signal.
In the case of a laser printer which obtains timing clock signals (image scan clock signals) for determining the print starting positions by frequency-dividing the master clock pulses, it is possible to match the print starting positions of the laser beams even when the mounting error corresponds to the time interval which is within one period of the master clock signal. However, the frequency of the image scan clock signals becomes in the order of 5 MHz in a high-speed laser printer. This means that the frequency of the master clock signal must be in the order of 50 MHz in order to control the print starting positions of the laser beams with a precision within 1/10 dot of the image. But it is impractical to use such a high frequency for the master clock signal.
As a conceivable method of not using the high-frequency master clock signal, it is possible to set the frequency of the master clock signal to a low frequency which is approximately the same as that of the image scan clock signals. In this case, a plurality of kinds of master clock signals are used having the phases thereof successively delayed by a constant value, and a most appropriate kind of master clock signal is selected depending on the output signal of the photodetector. It is possible to accurately control the print starting position of each laser beam without the use of the master clock signal having the high frequency. However, when overlapping a plurality of images such as in the case of a color printing, it is impossible to adjust the relative positions of the images, and an adjustment cannot be carried out to fully compensate for the mounting error corresponding to the time interval which is within one period of the master clock signal.